1. Field of the Invention
The present invention relates generally to semiconductor memory devices, and more particularly, to termination units for such memory devices.
2. Description of the Related Art
With increase in data transfer rate between a semiconductor memory device and a memory controller, interface techniques for reducing distortion of transferred data and for more accurate transfer of data are desired. Examples of such interface techniques include a low voltage transistor-transistor logic (LVTTL) interface technique and a stub-series terminated logic (SSTL) interface technique.
For the LVTTL interface technique, a synchronous dynamic random access memory (SDRAM) and a memory controller are directly connected to each other on a printed circuit board. The LVTTL interface technique is typically used for an SDRAM having an operating frequency of 100 MHz. On the other hand, for the SSTL interface technique, a termination circuit is formed on a main board for impedance matching between a transmission signal line and a semiconductor device connected to the transmission signal line. The SSTL interface technique is typically used for a double data rate (DDR) SDRAM having an operating frequency of 200 MHz or more.
However, when the aforementioned interface techniques are used for a memory device having an operating frequency of 400 MHz or more, signal distortion tends to occur on the data transfer lines. Thus, on-die termination (ODT) techniques are used for minimizing distortion of transmission signals by providing termination units at insides of a memory device and a memory controller. Different types of ODT are used depending on the kind of memory device to which the ODT is applied. For example, an open-drain type ODT is used in a graphic double data rate 3 (GDDR3) SDRAM and a rambus dynamic random access memory (RDRAM). On the other hand, a push-pull type ODT is used in a GDDR2 SDRAM.
The open-drain type ODT and the push-pull type ODT are now described with reference to FIGS. 1A, 1B, 2A, and 2B. FIGS. 1A and 1B are circuit diagrams with components for conventional termination units and input buffers. FIG. 1A shows an open-drain type termination unit 14 and an input buffer 11, and FIG. 1B shows a push-pull type termination unit 21, an input buffer 25, and an output buffer 26.
The open-drain type termination unit 14 of FIG. 1A advantageously has lower power consumption, but disadvantageously has higher signal distortion, compared to the push-pull type termination unit 21 of FIG. 1B. On the other hand, the push-pull type termination unit 21 of FIG. 1B is particularly advantageous for memory devices operating with higher frequencies because signal distortion is smaller, but disadvantageously has higher power consumption.
FIGS. 2A and 2B illustrate signals S1 and S2 output from the input buffers 11 and 25 of FIGS. 1A and 1B, respectively. Referring to FIGS. 2A and 2B, the output signal S1 of the input buffer 11 coupled to the termination unit 14 of the open-drain type is more distorted (i.e., has higher signal distortion) than the output signal S2 of the input buffer 25 coupled to the termination unit 21 of the push-pull type. The term signal distortion generally refers to an indication of a level of deviation of a signal from a desired waveform of the signal.
In the prior art, termination units of the same type are connected to all input signal lines and data input/output signal lines of conventional semiconductor memory devices. For example, in a semiconductor memory device requiring transmission signals with high quality, termination units of the push-pull type having higher power consumption but lower signal distortion are connected to all the input signal lines and the data input/output signal lines. On the other hand, in a semiconductor memory device requiring low power consumption, termination units of the open-drain type with lower power consumption but higher signal distortion are connected to all the input signal lines and the data input/output signal lines.
Typically, frequency components of the input signals of a semiconductor memory device are different depending on the kinds of the input signals. For example, data signals typically have double the frequency of command signals or address signals. However, semiconductor memory devices of the prior art disadvantageously use termination units of the same type for all the input signal lines and the data input/output signal lines. When termination units of the push-pull type are used for all the signal lines of a semiconductor memory device for lower signal distortion, power consumption is disadvantageously increased. On the contrary, when termination units of the open-drain type are used for all the signal lines for lower power consumption, signal distortion is disadvantageously increased.